(a) Field of the Invention
The present invention is concerned with a power amplifying circuit and more particularly, it relates to a power amplifier capable of changing over the supply voltage and thereby shifting the load line.
(B) Description of the Prior Art
Discussion will hereunder be made on the conventional emitter follower power amplifier by referring to FIG. 1. FIG. 1 shows a known single-ended push-pull power amplifying circuit of the emitter follower type. The respective emitters of the transistors Q.sub.1 and Q.sub.2 are connected to one end of a load resistor R.sub.L through respective resistors R.sub.el and R.sub.e2 having small resistances intended for the prevention of excessive current flow and for a stabilization of operation. The other end of the load R.sub.L is grounded. The respective collectors of these two transistors Q.sub.1 and Q.sub.2 are connected to a positive and a negative supply voltage source +V.sub.1 and -V.sub.1, respectively. The respective bases of the transistors Q.sub.1 and Q.sub.2 are applied with an input signal e.sub.i via a positive and a negative biasing voltage source +E.sub.b and -E.sub.b, respectively. In order to increase the peak output power in the above-mentioned power amplifying circuit, it is necessary to elevate the voltages of the supply voltage sources + V.sub.1 and -V.sub.1 in order to obtain a broader operative range. However, in case the voltages of the supply voltage sources +V.sub.1 and -V.sub.1 are elevated, the collector-emitter voltage between the transistors Q.sub.1 and Q.sub.2 relative to a certain load current will become higher, so that the collector loss, i.e., the power loss due to the collector current, will become increased.
Let us now refer to FIG. 2 which shows schematic characteristic curves of an amplifying transistor.
The ordinate represents the collector current and the abscissa represents the collector-to-emitter voltage. As shown in the figure, the current curves are assumed to take a constant value in the active region. Lines l.sub.1 and l.sub.2 represent examples of load line for a low and a high supply voltage V.sub.1 and V.sub.2. When the supply voltage is low at V.sub.1, the collector-to-emitter voltage is low and hence the power loss in the element is small. When a large input signal is supplied, however, the output current may be clipped at point F as shown by the dotted line on the left. To allow large signals to be amplified without distortion, the load line should be shifted rightwards. Namely, the supply voltage should be increased. The load line l.sub.2 represents the case under such increased supply voltage V.sub.2 with the same load resistance. In this case, the operative region may be expanded to DE compared to BA on the line l.sub.1. Thus, large signal amplification is made possible. In such case, however, the power loss in the element increases. Namely, for the similar output current I.sub.c, the power consumed in the element is I.sub.c .multidot.(V.sub.2) for the load line l.sub.2 in contrast to I.sub.c .multidot.V.sub. 1 for the load line l.sub.1. Thus, the power loss is increased by I.sub.c .multidot.(V.sub.2 -V.sub.1 ) for providing the same output current. In this way, according to the conventional power amplifier, large output current and low power consumption have been the contradicting problems.